This project is part of a long-term research program concerned with providing specific new diagnostic tests, and eventually therapies, for diseases and disorders of the vestibular system of the inner ear. Abnormal vestibular function produces vertigo, postural instability and patients with vestibular pathology may be very severely incapacitated. However clinical assessment of the functional status of the various vestibular sensory regions is difficult. Most clinical assessment techniques rely on measuring eye movement responses to acceleration stimulation of the inner ear; head movements and head position are detected by the vestibular receptors and result in characteristic reflexive compensatory eye movements. Some vestibular sensory regions have proved to be extremely difficult to assess in this way: in particular the linear acceleration sensors - the otoliths (the receptors of the utricular and saccular maculae in each labyrinth). We think we have found a new diagnostic indicator of the function of the otoliths - that they control the location of the axis of eye rotation. In experiments over the last 3 years at Sydney, Australia we have been using combinations of linear and angular accelerations generated by eccentric yaw rotation on a human centrifuge, together with some of the first recordings of human three-dimensional eye movement responses. We have found that compared with normal healthy subjects, patients with loss of one vestibular labyrinth have systematic differences in the location of the axis of eye rotation during such combination acceleration stimuli. We consider that those differences are primarily due to the absence of otolithic input from one side. On the basis of our own results, evidence from other studies on monkeys, and our own modelling work, we hypothesize that the otoliths exert a major control over the axis of eye rotation. This project is concerned with testing that hypothesis by determining the effect of varying head pitch and head roll positions on the axis of eye rotation during centered and eccentric yaw rotation. Normal healthy subjects and patients with known unilateral vestibular loss following vestibular nerve removal for the treatment of acoustic neuroma will receive controlled combinations of angular and linear accelerations and their eye movements will be measured in three dimensions during centered and then eccentric rotation. Our standard data analysis procedures will be used in order to measure the axis of eye rotation and determine whether it is affected by the various head pitch and roll positions. The location of the axis of eye rotation during angular acceleration may be a diagnostic indicator of otolith function but to establish that hypothesis we need to carry out these experiments in order to understand the relative role of the semicircular canals and the otoliths in the control of this axis. This project uses the unique facilities which we have developed and tested, the computer programs which we have written, tested and validated and our unique group of cooperative patients with known unilateral vestibular loss. It addresses an important question with urgent direct clinical relevance. Its outcome will be a new understanding of the vestibular control of the axis of eye rotation and hopefully new diagnostic tools for evaluating the functional status of the otoliths.